The present invention relates to a hydraulic power steering system for turning steerable wheels of a vehicle.
A known power steering gear for turning steerable wheels of a vehicle is hydraulically operated. The power steering gear is associated with a pump that may be powered by the vehicle engine. The power steering gear, to aid the vehicle driver in turning the steerable wheels of the vehicle, utilizes pressurized hydraulic fluid from the pump.
The power steering gear includes a hydraulic motor and a steering valve for controlling fluid flow to the hydraulic motor. The hydraulic motor includes two chamber portions that are separated by an axially movable piston. A fluid pressure difference in the respective chamber portions causes the piston to move axially resulting in movement of the steerable wheels of the vehicle. Resistance to movement of the steerable wheels results in an increased pressure in the inlet of the steering gear. This increased pressure may result in some instability in the vehicle steering and some audible noise.
The present invention is a hydraulic power steering system for turning steerable wheels of a vehicle. The system comprises a steering gear that includes a hydraulic motor and a steering valve for controlling actuation of the hydraulic motor by controlling fluid flow to the hydraulic motor. The hydraulic motor when actuated turns steerable vehicle wheels. The steering gear has an inlet, in communication with a fluid source, and an outlet for directing the fluid from the steering gear to a reservoir. The steering gear is susceptible to pressure fluctuations that may cause noise during steering.
The system further comprises a pressure relief valve having an unactuated condition and actuated conditions. The pressure relief valve, in the unactuated condition, restricts fluid flow through the pressure relief valve. In an actuated condition, the pressure relief valve permits fluid flow through the pressure relief valve. The pressure relief valve progressively moves from the unactuated condition to an actuated condition as a pressure differential between the inlet and the outlet of the steering gear increases. The pressure relief valve, when in an actuated condition, provides fluid communication between the inlet and the outlet of the steering gear to minimize pressure fluctuations in the steering gear.